Re: [BLACKBOX] Storing an extended object

From: [at]} <Josef>
Date: Tue, 11 Nov 2008 10:03:26 +0100

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Make the Externalize method EXTENSIBLE.
Then you can overwrite it in the sub class.
I would not recommend to handle sub classes in
the base class.

Note:
The general convention is, as far as I undestand it,
to (1) call the super method and (2) to store a version code
and (3) to store the data.
Storing a (one byte) version code gives you the opportunity to
change the persistent file format later without
invalidating any existing files.
Internalize would then of course have to support all
previous formats.
The storage overhead is acceptable.

- JT



----- Original Message -----
From: "Douglas G. Danforth" <danforth{([at]})nowhere.xy
To: <BLACKBOX{([at]})nowhere.xy
Sent: Tuesday, November 11, 2008 12:20 AM
Subject: [BLACKBOX] Storing an extended object


>I am not quite sure how to implement the Externalize and Internalize
> procedures
> for objects that are an extension of a base type. If I write an
> Externalize procedure
> (one for the base and one for the extension) then BB complains. So must
> I do something
> like the following?
>
> TYPE
> Base = POINTER TO EXTENSIBLE RECORD(Stores.Store)
> data: INTEGER
> END;
> Sub = POINTER TO RECORD(Base)
> moreData: INTEGER;
> END;
>
> PROCEDURE (base: Base) Externalize- (VAR wr: Stores.Writer);
> BEGIN
> wr.WriteInt(base.data);
> WITH base: Sub DO
> wr.WriteInt(base.moreData);
> ELSE
> END
> END Externalize;
>
> But now when I internalize what must I do?
>
> This seems cumbersome and not very elegant.
> -Doug Danforth

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Received on Tue Nov 11 2008 - 10:03:26 UTC